Traditionally, most exterior coatings have consisted of a single, pigmented coat. Automotive coatings are just one example. Currently, however, the auto industry uses basecoat/clearcoat technology. First, a relatively thin (0.6-0.8 mil) basecoat that incorporates the pigment or colored material is applied. The basecoat is then sealed by applying a thicker (1.8-2.2 mils) "clearcoat" that contains no pigment. Advantages of the modern approach include higher-solids, lower-VOC coatings; improved appearance; and improved weatherability. Because only a thin basecoat is needed, little pigment is used. The thick clearcoat has a wet, glossy look, and gives a feeling of depth. In addition, it shields the pigment with a thick, UV-resistant barrier. The clearcoat is applied to the basecoat without an intermediate baking step. This "wet-on-wet" method minimizes cost and promotes intercoat adhesion.
Exterior coatings, including automotive clearcoats, are often made by reacting a hydroxy-functional resin with a crosslinker. The resin is usually a hydroxy-functional acrylic resin, which imparts to the coating excellent durability and resistance properties. Melamines, silanes, and polyisocyanates are the most widely used crosslinkers. Acrylic-melamine coatings are used predominantly in automotive OEM applications, while acrylic-urethane coatings are used mostly in refinish ("after-market") applications.
Hydroxy-functional acrylic resins currently used in exterior coatings incorporate recurring units of hydroxyalkyl acrylates, ordinary acrylates, and optionally, other vinyl monomers such as styrene. Hydroxyalkyl acrylate monomers are reaction products of acrylic acid or methacrylic acid and an epoxide (e.g., ethylene oxide or propylene oxide). Unfortunately, hydroxyalkyl acrylates are much more expensive than ordinary acrylate monomers. In addition, a large proportion of hydroxyalkyl acrylate must be used to provide sufficient hydroxyl functionality for favorable resin reactivity with melamine and polyisocyanate crosslinkers. Thus, resins from hydroxyalkyl acrylate monomers are often expensive.
Increasingly stringent EPA air quality regulations limit volatile organic compound (VOC) emissions to the atmosphere. The coatings industry is responding with high-solids formulations. Unfortuately, it is difficult to increase coating solids levels from the current 50 wt. % to the desired level of 60-70 wt. % or more. Low-molecular-weight, low-viscosity resins help to achieve higher-solids formulations, but often at the expense of lost physical properties. Ways to achieve high solids levels without sacrificing coating properties are needed.
Another approach to VOC reduction or elimination is the development of powder coatings. Powder coatings may ultimately dominate the market for exterior coatings, including automotive coatings. Powder coatings typically comprise a blend of a "hard" acrylic polymer (Tg=75.degree.-90.degree. C.) and a "soft" acrylic polymer (Tg=0.degree.-30.degree. C.). The blend, which has a Tg of about 50.degree.-60.degree. C., avoids cold-flow at ambient temperature. Powder coatings cure at 150.degree.-180.degree. C., but are sufficiently stable at lower temperatures (50.degree.-80.degree. C.) to allow thermal processing of the formulations. A solvent is typically not used. Acrylic resins used in powder coatings derive from the expensive hydroxyalkyl acrylate monomers described above. Less expensive powder coating resins are needed.
Recently, we introduced low-molecular-weight, hydroxy-functional acrylic resins that have cost and performance advantages over current resins based on hydroxyalkyl acrylates (see U.S. Pat. Nos. 5,475,073, 5,480,943, and 5,525,693, the teachings of which are incorporated herein by reference). The resins are copolymers derived from an allylic alcohol or propoxylated allylic alcohol, an acrylate or methacrylate monomer, and optionally, one or more additional ethylenic monomers (e.g., styrene). The resins are uniquely prepared without a chain-transfer agent or reaction solvent, and are useful in many thermoset polymer applications, including thermoset polyesters, polyurethanes, crosslinked polymeric resins, melamines, alkyds, uralkyds, and epoxy thermosets. The low viscosity and high hydroxyl functionality of the resins prompted us to investigate their value in exterior coatings, including high-solids and powder coatings.